1. Field of the Invention
This invention relates to single mode optical fibers and, more particularly, to the design and fabrication of such fibers that are compliant with international standards regarding a variety of characteristics including bend loss, dispersion, mode-field diameter (MFD), and higher-order mode (HOM) suppression.
2. Discussion of the Related Art
In contrast with standard single mode optical fiber used, for example, in land line, undersea and metro systems, access fiber, which is typically located closer to the user, includes fiber-to the-home (FTTH), jumper cables, and FTTx fiber (e.g., fiber-to-the-curb, indoor wiring). Access fiber must not only interface in a low loss, reliable way with standard single mode fiber (SMF), which carries optical signals to the location being accessed (e.g., home, business, or other facility), but also must be relatively insensitive to the effects of bending, which is inherent in many of the access fiber applications.
Thus, in access fiber applications it is highly desirable to have fibers that combine low bend loss and good compatibility with existing infrastructure and standards—in particular G.657 requirements with respect to bend loss and concurrently G.652 requirements with respect to a variety of characteristics, such as dispersion, HOM cutoff and MFD.
However, there is an inherent difficulty in achieving low bend loss without sacrificing properties important to compatibility, especially mode size, splice or connector loss, cutoff, and HOM suppression.
Ring-assisted or resonance-assisted fiber (RAF) designs alleviate these difficulties, but many previous RAF designs suffer from fabrication constraints and a lack of compliance with both G.657 and G.652 concurrently (e.g., G.657.B3 and G.652.D). Fabrication constraints lead to higher cost and smaller preform size. In particular, the interior region (i.e., excluding the outer cladding) of a conventional RAF has a refractive index profile fabricated using conventional vapor deposition techniques (e.g., MCVD). The various portions of the interior region (e.g., core, trench, ring/pedestal) have different refractive indices, which can be adjusted by doping with, for example, fluorine or creating hollow voids to produce a depressed-index region, or germanium to produce a raised-index region. Due to the large radial extent (cross-sectional area or volume) of the interior-region of a conventional RAF compared to a conventional single mode fiber, a significant fraction of the fiber volume is deposited using the vapor-phase process. Since the deposition rate of such processes is relatively slow, this type of fiber material has relatively low throughput and hence relatively high cost.
Therefore, there is a need for a RAF design that allows at least a portion of the interior-region to be fabricated by a technique other than conventional, low-deposition-rate vapor deposition.
In addition to manufacturing cost, some known RAFs have sufficiently low bend loss to be G.657.B3 compliant but fail to satisfy one or more of the requirements of G.652.D (such as its dispersion requirements). Therefore, there is also a need for a RAF design that is compliant with both G.657.B3 and G.652.D concurrently.